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The I method used to estimate the size of solar water heating system needed to achieve an 80% solar fraction is detailed in this pdf
How to size your system in detail using the RETScreen software...
Basically I used the RETScreen software to size solar water heating systems that would deliver an 80% solar fraction in fifteen US cities that cover all types of climates in the US. This basically just requires trying increasing amounts of collector area until the system provides an 80% solar fraction for the climate being looked at. Additional runs were made to see what happens as the collector area is increased or decreased from the area required for an 80% solar fraction.
RETScreen was developed by Natural Resources Canada and is a highly respected solar water heating analysis and sizing program. Some US states require that a RETScreen analysis be done before paying incentives for solar water heating systems.
RETScreen takes into account: local climate, collector type and efficiency, collector tilt and azimuth, ground water temperature, type and cost of backup water heating, water heating target temperature, hot water demand, storage tank size, and various other losses. It reports on energy produced, CO2 emissions, and provides a financial analysis including payback period and return on investment.
The RETScreen software is free and can be downloaded here...
Assumptions Used
In order to keep things managable, the following assumptions were made in the RETScreen analyses I did:
- Hot water demand is 48 gal per day which is typical for a family of three
- Collector efficiency curve parameters are based on the SRCC efficiency curve for a commercial flat plate collector with a black painted (non-selective) absorber.
- Collector tilt is set to the latitude of each city modeled, and the collector faces south.
- Cost is based on this cost table for the $2K system with the space heating left out. The cost is adjusted up or down depending on the collector and tank size used.
- These systems are DIY, so only the cost of materials are included.
- The existing water heating (before solar) is assumed to be electric at 12 cents per KWH.
See the description of the $2K System for the details on the collector, tank and controlls assumed for this system.
The tank size is done at 2.5 gallons per sqft of collector, but is not allowed to go below 100 gallons in order to have room for the pipe coil heat exchanger. It could be argued that the tank capacity per sqft of collector should depend on climate (larger for warmer climates) -- I looked at this a bit and it did not make much difference on DIY system cost or savings or performance.
The cost of the system is calculated as:
Total Cost = (Collector Area sqft)($8.5) + (Tank Cap gals)($1.8) + $426
So, a system with 60 sqft of collector and a tank size of (60 sqft)(2.5 gals/sqft) =
Where the $426 covers cost of plumbing, pump, potable water heat exchanger, controller, ...
All of these costs are for materials only with you supplying the labor.
If these assumptions do no match your situation, see this page...
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